Electrolytic method for the manufacture of hypochlorites

ABSTRACT

Hypochlorites, such as alkali metal hypochlorites, are made by electrolyzing brine in a cell having three or more compartments or zones therein, wherein anode and cathode compartments are separated from at least one intervening buffer compartment by cation-active permselective membranes of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether or of a sulfostyrenated fluorinated ethylene propylene polymer or by such a permselective membrane on the cathode side plus a porous asbestos diaphragm, while feeding chlorine gas to the buffer zone at such a rate and under such conditions as to produce hypochlorite therein. The hypochlorite may be converted to chlorate externally of the cell or, in a variation of the process, may be converted to chlorate in the buffer compartment.

United States Patent 1191 Eng et al. 5] Dec. 9, 1975 [5 ELECTROLYTICMETHOD FOR THE 3,853,720 12/1974 Korach et al. 204/98 MANUFACTURE OFHYPOCHLORITES 3,853,721 l2/l974 Darlington et al i 204/98 [75]Inventors: Jeffrey D. Eng North Vancouver P nmary Exam1ner-F. C.Edmundson 5:2 :5 Harke Burnaby both of Attorney, Agent, or F1'rmPeter F.Casella 731 Assignee: Hooker Chemicals & Plastics ABSTRACT Corporation,Niagara Falls, NY. Hypochlorites, such as alkali metal hypochlorites,are [22] Fied: N0 1, 1973 made by electrolyzing brine in a cell havingthree or more compartments or zones therein, wherein anode PP NO 411,620and cathode compartments are separated from at least one interveningbuffer compartment by cation-active [52] s CL H 204/95; 204/257. 204/265permselective membranes of a hydrolyzed copolymer 2 y I I I oftetrafluoroethylene and a fluorosulfonated per- [5l] Int. Cl. i C2581/26, COlB 11/06,

C258 8/08; 301K 1/00 fluorovinyl ether or of a sulfostyrenatedfluorinated 58 Field of Search A. 204/86, 87, 92, 93, 295, EthylenePmPYlene Plymer by Such a Permselec 204/296 tive membrane on the cathodeside plus a porous asbestos diaphragm. while feeding chlorine gas to the[56] References Cited buffer zone at such a rate and under suchconditions as to produce hypochlorite therein. The hypochlorite UNITEDSTATES PATENTS may be converted to chlorate externally of the cell or,:gxzg; g 2/ in a variation of the process, may be converted to 1ce .13,438,879 4/1969 Kircher et almr 204/95 chlorate m the buffercompartment 3,852,l35 12/1974 Cook et al. 4. 204/296 x 8 Claims, 3Drawing Figures MOCI MCI MCIO; MCI

US. Patent Dec. 9, 1975 Sheet 2 Of3 3,925,174

BRINE M610 (MX) MC! HYDROGEN 1:17;:111'5'5 Q51 5 ff-3E5: MOH

I5] 27 23 K25 29 I? WATER FIG. 2

ELECTROLYTIC METHOD FOR THE MANUFACTURE OF HYPOCHLORITES This inventionrelates to the electrolytic manufacture of hypochlorites. Morespecifically, it is of a process for making alkali metal hypochloritefrom chlorine and aqueous alkali metal hydroxide solution, both of whichreactants are produced in an electrolytic cell containing anode, cathodeand buffer compartments, with means provided for separating the cathodeand buffer compartments being a cation-active permselective membranewhich is a hydrolyzed polymer of a perfluorinated hydrocarbon and afiuorosulfonated perfiuorovinyl ether or a sulfostyrenatedperfluorinated ethylene propylene polymer.

Such cation-permeable membranes permit flow of hydroxyl ion from thecatholyte to the buffer zone but do not allow chloride ion to passthrough and to mix with the hydroxyl in the cathode compartment. Thus,when chlorine is added to a buffer compartment, hypochlorite is producedtherein, consuming the hydroxyl ion and preventing it from flowing tothe anolyte and at the same time a chloride-free alkali metal hydroxideis made in the cathode compartment.

Chlorine and caustic, essential and very large volume chemicals requiredby all industrial societies, are commercially produced by theelectrolysis of aqueous salt solutions. Improved electrolytic methodsutilize dimensionally stable anodes, which include noble metals, alloysor oxides or mixtures thereof on valve metals. The concept of employingpermselective diaphragms to separate anolyte from catholyte duringelectrolysis is not a new one and plural compartment electrolytic cellshave been suggested in which one or more of such membranes is employed.Recently, improved membranes which are of a hydrolyzed copolymer of aperfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl etherhave been described and in some experiments these have been used as themembranes between the catholyte and buffer zones of chlorine-causticcells. Such membranes have been further improved by surface treatments,preferably by modifications of the sulfonic group, to make them moreconductive and efficient. Also, sulfostyrenated perfluorinated ethylenepropylene polymers have been made into useful membranes.

Although the electrolysis of aqueous salt solutions is a technologicallyadvanced field of great commercial interest in which much research isperformed, and the importance of improving manufacturing methods thereinis well recognized, before the present invention the process thereof hadnot been practiced and the advantages of it had not been obtained.

In accordance with the present invention a method of electrolyticallymanufacturing a hypochlorite comprises electrolyzing an aqueous solutioncontaining chloride ions in an electrolytic cell having at least threecompartments therein, an anode, a cathode, at least one cation-activepermselective membrane selected from the group consisting of ahydrolyzed copolymer of a perfluorinated hydrocarbon and afluorosulfonated perfluorovinyl ether, and of a sulfostyrenatedperfluorinated ethylene propylene polymer, defining a cathodeside wallof a buffer compartment between the anode and cathode, an anode-sidewall of said buffer compartment being defined by such a cationactiveperselective membrane or a porous diaphragm, and such walls, with wallsthereabout, defining anode, buffer and cathode compartments, whilefeeding gaseous chlorine into the buffer compartment and regulating therate of feed thereof and reaction conditions to produce hypochlorite inthe buffer compartment. In a preferred embodiment of the invention thepermselective membranes are of a hydrolyzed copolymer oftetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of theformula FSO CF CF OCF(CF,)CF,OCF=CF which has 0 an equivalent weight ofabout 900 to 1,600, at least two such membranes are employed, at leastone of which separates the anolyte and buffer compartments and the otherof which separates the catholyte and buffer compartments, and themembranes are mounted on networks of supporting materials such aspolytetrafluoroethylene or asbestos filaments. In some preferred aspectsof the invention the hypochlorite is converted to chlorate, eitherexternally or internally of the cell. The described preferred copolymersmay be further moditied to improve their activities, as by surfacetreating. modifying the sulfonic group or by other such mechanism. Suchvarieties of the polymers are included within the generic descriptiongiven.

The invention will be more readily understood by reference to thefollowing descriptions of embodiments thereof, taken in conjunction withthe drawing of apparatuses and means for effecting the inventedprocesses In the drawing:

FIG. 1 is a schematic representation of the arrangement of equipment forproducing hypochlorite is ar electrolytic cell by a method of thisinvention and subsequently converting it to chlorate outside the cell;

FIG. 2 is a schematic view of an electrolytic cell ir which chlorate isproduced internally; and

FIG. 3 is a schematic view of apparatus like that 0. FIG. 1, includingmeans to remove chloride from the chlorate made and to recirculatechlorate through the cell buffer compartment to increase chloratecontent ir the product stream.

In the FIGURES, to facilitate understanding of the process, the flows oftypical and preferred reactant: and products are illustrated. M standsfor alkali metal preferably sodium, but other halide-forming cation: mayalso be employed and in some instances bromim may be at least partiallysubstituted for chlorine.

In FIG. 1 electrolytic cell 11 includes outer wall 13 anode 15, cathodel7 and conductive means 19 and 21 for connecting the anode and thecathode to sources 0 positive and negative electrical potentials,respectively inside the walled cell cation-active permselective membranes 23 and 2S divide the volume into anode or ano lyte compartment27, cathode or catholyte compart ment 29 and buffer compartment 31. Anacidic aque ous solution 32 of alkali metal halide is fed to the anolyte compartment through line 33 and chlorine gas i fed to the buffercompartment through line 35. Recir culated buffer solution may also befed into the buffe compartment, through a separate line, 36 or a com monline with the chlorine or water. Water may be ad mitted through line 36to maintain the desired liqui level in the buffer compartment. Ofcourse, liquid lex els should be maintained in all compartments and thisi often effected with known feed-overflow technique: the apparatus foreffecting which is known and there fore, is not illustrated.

Halogen, e.g., chlorine gas, is removable from the ar olyte compartmentthrough line 34 and aqueous sr dium hydroxide is removable from thecatholyte con partment through line 37. An aqueous solution of alkalimetal hypochlorite, with some dissolved alkali metal chloride, isremovable through line 39 and may be passed through that line toreaction vessel or mixer 41 in which it is mixed with halogen, e.g.,chlorine gas, from line 34. The chlorine passes through line 43, and maybe pulled through that line by low pressure created by pumpinghypochlorite-chloride or hypochloritechlorate-chloride solution 45through line 47, pump 49 and return line 51, through eductor reactor 53,in which intimate mixing is effected. Chlorine gas in the upper portionof reaction and retention vessel 41 may be vented off or may berecycled, too, The chlorate made is removed as an aqueous solution, withalkali metal chloride, through discharge line 55. The chlorate-chloridesolution may be circulated through lines 47 and SI, pump 49, reactor 53and vessel 41 until the chlorate-chloride concentration is increased toa useful level. Chloride may be removed by precipitation and if desired,chlorate may be crystallized out by installation of the appropriateapparatus in lines 47 and 51. Be cause sodium chloride is relativelyinsoluble, compared to sodium chlorate, it should be removed beforechlorate crystals are manufactured; otherwise chloride solids can blockorifices, etc., during manufacturing.

In some aspects of the invention, when it is preferred to produce thehypochlorite for direct use, it is removed through line 39, togetherwith alkali metal chloride. Some of it may subsequently be converted tochlorate. Hydrogen is obtainable from line 40.

In the operation of the invented process chlorine is generated at theanode and alkali metal hydroxide and hydrogen are produced at thecathode. The normal tendency for alkali metal halide to move into thecatholyte and increase the halide content of the hydroxide made iscounteracted by the cationic permselective membrane 25 and thisprevention of chloride flow is aided by the presence of the additionalpermselective membrane 23. Yet, alkali metal hydroxide may migrate fromcatholyte to the anolyte in ordinary cells and such migration caninterfere with the caustic or sodium ion current efficiency if theproduct made is useless or is not recovered. Caustic, sodium ion orcathode current efficiency is the percentage of useful product made,compared to 100% maximum, with the current flow employed. Sodium ionefficiency may be the most exact of the terms employed but all are used.Thus, if sodium hydroxide is chemically reacted to make recoverablesodium hypochlorite or sodium chlorate, coulombs are not wasted, as theyare when hydroxyl ions are electrolytically converted to useless oxygenat the anode. Anode or chlorine efficiencies are figured in the samegeneral way.

In the present cell the addition of chlorine to the buffer zone causesthe alkali metal hydroxide migrating through the membrane, asillustrated, to be converted to hypochlorite and chloride, which do notpass through the permselective membranes. Therefore, the processsatisfactorily produces a chloride-free caustic at satisfactory highcurrent efficiency and additionally makes a desired byproduct, thehypochlorite, which may be further converted to chlorate, when desired.

In FIG. 2 the manufacture of chlorate in the buffer zone is shown, usinga cell like that of FIG. 1. The only difference in operation is in theemployment of sufficient chlorine to diminish the pH further, favoringformation of chlorate rather than of hypochlorite, which is normallyproduced at a higher pH. Acids and bases may also be used to regulatethe pH. A liquid medium such as recirculated buffer solution or otherchloratechloride-water solution may be added to the buffer zone throughline 36 so as to help control the tempera ture, and sometimes, toincrease the percentage of chlorate in the buffer zone and in therecirculating liquid to such a level that after removal of chloride,chlorate may be crystallized out.

In FIG. 3 external manufacture of chlorate is illustrated, with buildupof chloride and chlorate concentrations by recirculation, followed byremoval of the chloride, which may then be followed by crystallizationof the chlorate. As is illustrated, chloridechlorate solution may berecirculated through vessel 41 via lines 47 and 51 with the solutionpassing through pump 49 and reactor 53. During recirculation additionalreaction with MOCl from the cell is effected in reactor 53 and theconcentrations of the hypochlorite and chloride resulting from suchreaction are increased. Because the chloride is less soluble and isproduced to a greater extent, it will soon crystallize out in thereactor or retention vessel, causing processing difficulties.Accordingly, it is removed in separator or crystallizer 61 and morepure, more concentrated chlorate is continually circulated andultimately, is drawn off from the retention vessel 41, possibly forfurther concentration and- /or crystallization out as the solid.Atjunction 63 a proportioning valve may be located and the concentrationof chlorate in the circulating system may be further increased byreturning a proportion of it through line 65 to cell 11. Desired pHs atvarious parts of the system may be controlled by regulating theproportions of chlorine utilized at such different locations.

Although some circulations and recirculations of materials of theprocess are illustrated, others may also be effected. Thus, anolyte,buffer compartment solution and catholyte recirculation may be utilizedto maintain the various solutions at the same concentrations throughouttheir respective compartments. Alternatively, once-through processes andhybrid" processes are also useful. Similarly, recirculation ofchloratechloride solutions may be to the anolyte compartment, at leastin part, to convert the chloride thereof to chlorine and thus reduce theconcentration of it in the chlorate-chloride mixtures.

In the preferred embodiments of the invention the buffer zone orcompartment has two opposing boundaries or walls thereof, dividing itfrom anode and cathode compartments, respectively, both of the describedhydrolyzed copolymer membranes, usually supported on an open network,screen or cloth of electrolyteand product-resistant material which ispreferably filamentary in form. The cationic membranes oppose or preventthe passage of anions such as halide, hypohalite and halate ions, whileallowing the passage of cations, e.g., alkali metal and hydrogen ions.Low molecular weight anions, such as hydroxyl, may also pass through thecationic membranes.

The selective ion-passing effects of cationic membranes have been notedin the past but the membranes of this invention have not been employedin the present processes before and their unexpectedly beneficialeffects have not been previously obtained or suggested. Thus, with theuse of a comparatively thin membrane, preferably supported as describedherein, several years of operation under commercial conditions areobtainable without the need for removal and replacement of the membrane,while all the time it efficiently prevents undesirable migration ofhypochlorite from the buffer compartment and prevents the chloride ionsof the anolyte from entering the buffer compartments, while alsostopping any chloride in the buffer zone from transferring to thecatholyte. Together with the use of the buffer zone between the anolyteand catholyte zones, it prevents hydrogen formed on the cathode sidefrom escaping into the halogen formed on the anode side. In this respectthe present membranes are superior to prior art membranes because theyare more impervious to the passage of hydrogen, even in comparativelythin films, than are various other polymeric materials. Also importantis their ability to prevent transfer of chlorine gas into the hydrogenproduced at the cathode, especially when chlorine is fed to the buffercompartment, since when chlorine is present in hydrogen an explosivemixture may be formed. The superiority of the preferred membranes of thedescribed copolymer (including modified or surface treated versionsthereof) over the prior art membranes in the various described aspectsis also evident, usually to a lesser degree, in the sulfostyrenatedfluorinated ethylene propylene polymers.

Although the preferred embodiments of the invention utilize a pair ofthe described membranes to form the three compartments of the presentcells it will be evident that a greater number of compartments, e.g., 4to 6, including plural buffer zones, may be employed. Similarly, also,while the compartments will usually be separated by flat membranes andwill usually be of substantially rectilinear or parallelepipedalconstruction, various other shapes, including curves, e.g., ellipsoids,irregular surfaces, e.g., sawtoothed or plurally pointed walls, may alsobe utilized. In another variation of the invention the buffer zone(s),formed by the plurality of membranes, will be between bipolarelectrodes, rather than the monopolar electrodes which are describedherein. Those of skill in the art will know the variations in structurethat will be made to accommodate bipolar, rather than monopolarelectrodes, and therefore, these will not be described in detail. Ofcourse, as is known in the art, pluralities of the individual cells willbe employed in multi-cell units, often having common feed and productmanifolds and being housed in unitary structures. Again, suchconstructions are known to those in the art and need not be discussedherein.

For most satisfactory and efficient operations the volume of the buffercompartment(s) will usually be from I to 100%, preferably from to 70%that of the sum of the volumes of the anode and cathode compare ments.

Although the utilization of the present cationic or cation-activemembranes to define the buffer compartment(s) and separate it/them fromthe anolyte and catholyte sections is highly preferred it is possible tooperate with a conventional diaphragm separating the anode compartmentfrom the buffer compartment. However, the membrane will be employed toseparate the catholyte from the buffer zones in order to produce thehighly desirable salt-free caustic. Otherwise, even if such a membranewas employed to separate the anolyte from the buffer zone, halidepresent in the buffer section due to addition of brine or production bythe reaction of chlorine with the caustic to form hypochlorite, couldpass through the diaphragm to contaminate the caustic. In manyapplications salt-free caustic is highly desirable and therefore,3-compartment cell structures having a cathode-side porous diaphragm,such as illustrated in the U.S. Environmental Protection Agencypublication entitled Hypochlorite Generator for Treat ment of CombinedSewer Overflows (Water Pollutior Control Research Series 1 1023 DAA03/72) are unsat isfactory. Additionally, the conventional diaphragmswhich are usually of deposited asbestos fibers, tend tc become blockedwith insoluble impurities from the brine and have to be cleanedperiodically, usually necessitating shutdown of the cell and often,replacement of the diaphragm.

The aqueous solution containing chloride ions is normally a watersolution of sodium chloride, although potassium and other solublechlorides, e.g., magnesiurr chloride and ammonium chloride, may beutilized, a least in part. However, it is preferable to employ thealkali metal chlorides and of these sodium chloride is the best. Sodiumand potassium chlorides include cation: which form soluble salts orprecipitates and which pro duce stable hydroxides. The concentration ofsodiurr chloride in a brine charged will usually be as high a: feasible,normally being from 200 to 320 grams pei liter for sodium chloride andfrom 200 to 360 g./l. f0] potassium chloride, with intermediate figuresfor mix tures of sodium and potassium chlorides. The electro lyte may beneutral or acidified to a pH in the range 0' about 2 to 6, acidificationnormally being effected, witl a suitable acid such as hydrochloric acid.Charging o the brine is to the anolyte compartment. The solid so diumchloride added to the liquid medium in the ano lyte results in a sodiumchloride concentration frorr 200 to 320 g./l. and most preferably of 250to 300 g./l ln recycle charges to the buffer compartment, if uti lized,the concentration will normally be less than 50 0 g./l., althoughchlorate contents may be higher and usually the chloride contents of thebuffer liquid: will be less than such limits, too.

Water may be charged to the buffer compartmen and in some cases it maybe desirable to charge wate with brine to the anolyte compartment.Dilute caustii may be recirculated to the catholyte compartment bu thisis not usually done. For the most part the llqLllt level in that zone ismaintained by transfer to it of mate rial(s) charged to the anolyteand/or buffer zone, plu water.

The presently preferred cation permselective mem brane is of ahydrolyzed copolymer of perfluorinatei hydrocarbon and afluorosulfonated perfluoroviny ether. The perfluorinated hydrocarbon ispreferabl tetrafluoroethylene, although other perfluorinated antsaturated and unsaturated hydrocarbons of 2 to 5 car bon atoms may alsobe utilized, of which the monoole finic hydrocarbons are preferred,especially those of to 4 carbon atoms and most especially those of 2 tocarbon atoms, e.g., tetrafluoroethylene, hexafluorop ro pylene. Thesulfonated perfluorovinyl ether which i most useful is that of theformula FS0,CF CF OCF(CF )CF,OCF=CF,. Such a material, named aperfluoro[2-(2-fluorosulfonylethoxy)-propyl viny ether], referred tohenceforth as PSEPVE, may b modified to equivalent monomers, as bymodifying th internal perfluorosulfonylethoxy component to the C0)responding propoxy component and by altering th propyl to ethyl orbutyl, plus rearranging positions c substitution of the sulfonyl thereonand utilizing isr mers of the perfluoro-lower alkyl groups, respectivelyHowever, it is most preferred to employ PSEPVE.

The method of manufacture of the hydrolyzed cc polymer is described inExample XVI] of US. Pat. Nt

3,282,875 and an alternative method is mentioned in Canadian Pat. No.849,670, which also discloses the use of the finished membrane in fuelcells, characterized therein as electrochemical cells. The disclosuresof such patents are hereby incorporated herein by reference. In short,the copolymer may be made by reacting PSEPVE or equivalent withtetrafluoroethylene or equivalent in desired proportions in water atelevated temperature and pressure for over an hour, after which time themix is cooled. It separates into a lower perfluoroether layer and anupper layer of aqueous medium with dispersed desired polymer. Themolecular weight is indeterminate but the equivalent weight is about 900to 1,600 preferably 1,100 to 1,400 and the percentage of PSEPVE orcorresponding compound is about to 30%, preferably to and mostpreferably about 17%. The unhydrolyzed copolymer may be compressionmolded at high temperature and pressure to produce sheets or membranes,which may vary in thickness from 0.02 to 0.5 mm. These are then furthertreated to hydrolyze pendant SO F groups to SO H groups, as by treatingwith 10% sulfuric acid or by the methods of the patents previouslymentioned. The presence of the SO H groups may be verified by titration,as described in the Canadian patent. Additional details of variousprocessing steps are described in Canadian Pat. No. 752,427 and US. Pat.No. 3,041,317, also hereby incorporated by reference.

Because it has been found that some expansion accompanies hydrolysis ofthe copolymer it is preferred to position the copolymer membrane afterhydrolysis onto a frame or other support which will hold it in place inthe electrolytic cell. Then it may be clamped or cemented in place andwill be true, without sags. The membrane is preferably joined to thebacking tetrafluoroethylene or other suitable filaments prior tohydrolysis, when it is still thermoplastic; and the film of copolymercovers each filament, penetrating into spaces between them and evenaround behind them, thinning the films slightly in the process, wherethey cover the filaments.

The membrane described is far superior in the present processes to allother previously suggested membrane materials. It is more stable atelevated temperatures, e.g., above 75C. It lasts for much longer timeperiods in the medium of the electrolyte and the caustic product anddoes not become brittle when subjected to chlorine at high celltemperatures. Considering the savings in time and fabrication costs, thepresent membranes are more economical. The voltage drop through themembranes is acceptable and does not become inordinately high, as itdoes with many other membrane materials, when the caustic concentrationin the cathode compartment increases to above about 200 g./l. ofcaustic. The selectivity of the membrane and its compatibility with theelectrolyte does not decrease detrimentally as the hydroxylconcentration in the catholyte liquor increases, as has been noted withother membrane materials. Furthermore, the caustic efficiency of theelectrolysis does not diminish as significantly as it does with othermembranes when the hydroxyl ion concentration in the catholyteincreases. Thus, these differences in the present process make itpracticable, whereas previously described processes have not at tainedcommercial acceptance. While the more preferred copolymers are thosehaving equivalent weights of 900 to 1,600, with 1,100 to 1,400 beingmost preferred, some useful resinous membranes produced by the presentmethod may be of equivalent weights from 500 to 4,000. The mediumequivalent weight polymers are preferred because they are ofsatisfactory strength and stability, enable better selective ionexchange to take place and are of lower internal resistances, all ofwhich are important to the present electro-chemical cell.

Improved versions of the above-described copolymers may be made bychemical treatment of surfaces thereof, as by treatments to modify theSO,H group thereon. For example, the sulfonic group may be altered ormay be replaced in part with other moieties. Such changes may be made inthe manufacturing process or after production of the membrane. Wheneffected as a subsequent surface treatment of a membrane the depth oftreatment will usually be from 0.001 to 0.01 mm. Caustic efficiencies ofthe invented processes, using such modified versions of the presentimproved membranes, can increase about 3 to 20%, often about 5 to 15%.Exemplary of such treatments is that described in French Pat. No.2,152,194 of Mar. 26, 1973 in which one side of the membrane is treatedwith NI-I to form SO Nl-l groups.

In addition to the copolymers previously discussed, includingmodifications thereof, it has been found that another type of membranematerial is also superior to prior art films for applications in thepresent processes. Although it appears that tetrafluoroethylene (TFE)polymers which are sequentially styrenated and sulfonated are not usefulfor making satisfactory cationactive permselective membranes for use inthe present electrolytic processes it has been established thatperfluorinated ethylene propylene polymer (FEP) which is styrenated andsulfonated makes a useful membrane. Whereas useful lives of as much as 3years or more (that of the preferred copolymers) may not be obtained thesulfostyrenated FEP's are surprisingly resistant to hardening andotherwise failing in use under the present process conditions.

To manufacture the sulfostyrenated FEP membranes a standard FEP, such asmanufactured by E. I. DuPont de Nemours & Co. Inc., is styrenated andthe styrenated polymer is then sulfonated. A solution of styrene inmethylene chloride or benzene at a suitable concentration in the rangeof about 10 to 20% is prepared and a sheet of PEP polymer having athickness of about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm., isdipped into the solution. After removal it is subjected to radiationtreatment, using a cobalt radiation source. The rate of application maybe in the range of about 8,000 rads/hr. and a total radiationapplication is about 0.9 megarads. After rinsing with water the phenylrings of the styrene portion of the polymer are monosulfonated,preferably in the para position, by treatment with chlorosulfonic acid,fuming sulfuric acid or 80,. Preferably, chlorosulfonic acid inchloroform is utilized and the sulfonation is completed in about hour.

Examples of useful membranes made by the described process are productsof RAI Research Corporation, I-lauppauge, New York, identified as18ST12S and 16ST13S, the former being 18% styrenated and having of thephenyl groups monosulfonated and the latter being 16% styrenated andhaving 13/16 of the phenyl groups monosulfonated. To obtain 18%styrenation a solution of 17-56% of styrene in methylene chloride isutilized and to obtain the 16% styrenation a solution of 16% of styrenein methylene chloride is employed.

The products resulting compare favorably with the preferred copolymerspreviously described, giving voltage drops of about 0.2 volt each in thepresent cells at a current density of 2 amperes/sq. in., the same as isobtained from the copolymer.

The membrane walls will normally be from 0.02 to 0.5 mm. thick,preferably from 0.1 to 0.5 mm. and most preferably 0.1 to 0.3 mm. Whenmounted on a polytetrafluoroethylene, asbestos, titanium or othersuitable network, for support, the network filaments or fibers willusually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15mm., corresponding to up to the thickness of the membrane. Often it willbe preferable for the fibers to be less than half the film thickness butfilament thicknesses greater than that of the film may also besuccessfully employed, e.g., 1.1 to 5 times the film thickness. Thenetworks, screens or cloths have an area percentage of openings thereinfrom about 8 to 80%, preferably 10 to 70% and most preferably 30 to 70%.Generally the cross sections of the filaments will be circular but othershapes, such as ellipses, squares and rectangles, are also useful. Thesupporting network is preferably a screen or cloth and although it maybe cemented to the membrane it is preferred that it be fused to it byhigh temperature, high pressure compression before hydrolysis of thecopolymer. Then, the membrane-network composite can be clamped orotherwise fastened in place in a holder or support. it is preferred toemploy the described backed membranes as walls of the cell between theanolyte and catholyte compartments and the buffer compartment(s) but ifdesired, that separating the anolyte and buffer compartments may be ofconventional diaphragm material, e.g., deposited asbestos fibers orsynthetic polymeric fibrous material (polytetrafluoroethylene,polypropylene). Also, treated asbestos fibers may be utilized and suchfibers mixed with synthetic organic polymeric fibers may be employed.However, when such diaphragms are used efforts should be made to removehardness ions and other impurities from the feed to the cell so as toprevent these from prematurely depositing on and blocking thediaphragms.

The material of construction of the cell body may be conventional,including concrete or stressed concrete lined with mastics, rubbers,e.g., neoprene, polyvinylidene chloride, FEP, chlorendic acid basedpolyester, polypropylene, polyvinyl chloride, TFE or other suitableplastic or may be similarly lined boxes of other structural materials.Substantially self-supporting structures, such as rigid polyvinylchloride, polyvinylidene chloride, polypropylene or phenol formaldehyderesins may be employed, preferably reinforced with moldedin fibers,cloths or webs.

The electrodes of the cell can be made of any electrically conductivematerial which will resist the attack of the various cell contents. ingeneral, the cathodes are made of graphite, iron, lead dioxide ongraphite or titanium, steel or noble metal, such as platinum, iridium,ruthenium or rhodium. Of course, when using the noble metals, they maybe deposited as surfaces on conductive substrates, e.g., copper, silver,aluminum, steel, iron. The anodes are also of materials or have surfacesof materials such as noble metals, noble metal alloys, noble metaloxides, noble metal oxides mixed with valve metal oxides, e.g.,ruthenium oxide plus titanium dioxide, or mixtures thereof, on asubstrate which is conductive. Preferably, such surfaces are on or witha valve metal and connect to a conductive metal, such as thosepreviously described. Especially useful are platinum, platinum ontitanium, platinum oxide on titanium, mixtures of ruthenium and platinumand their oxides on titanium and similar surfaces on other valve metals,e.g., tantalum. The conductors for such materi als may be aluminum,copper, silver, steel or iron, with copper being much preferred. Apreferable dimensionally stable anode is ruthenium oxide-titaniumdioxide mixture on a titanium substrate, connected to a copperconductor.

The voltage drop from anode to cathode is usually in the range of about2.3 to 5 volts, although sometimes it is slightly more than 5 volts,e.g., up to 6 volts. Preferably, it is in the range of 3.5 to 4.5 volts.The current density, while it may be from 0.5 to 4 amperes per squareinch of electrode surface, is preferably from 1 to 3 amperes/sq. in. andideally about 2 amperes/sq. in. The voltage ranges given are forperfectly aligned electrodes and it is understood that where suchalignment is not exact, as in laboratory units, the voltages can be upto about 0.5 volt higher.

The feeding of gaseous chlorine into the buffer compartment is at such arate as to enable it to react with the sodium hydroxide entering suchcompartment from the catholyte and to convert substantially all of it tobypochlorite (or further, to chlorate), thereby preventing it frommigrating further into the anolyte. It will be evident that the rate offeed is controlled in response to variations in caustic transmissioninto the buffer compartment. Additions may be in response to pHfluctuations in the buffer zone. Normally, to produce hypochlorite,possibly with some chlorate therein, in the buffer zone, a pH of 8 to 11will be maintained whereas to produce chlorate therein this will belowered to 6 to 7.5, preferably 6 to 7. Control of the pH may be andpreferably is by chlorine addition but other acidifying agents may beemployed, also. On the average, it is considered that from 5 to 20% ofthe caustic produced in the catholyte compartment migrates to the buffercompartment and therefore, the stoichiometric amount of chlorine toconvert this caustic to hypochlorite will be employed, plus an excesswhen desired, e.g., from 5 to 20% of chlorine, to adjust the pH. Inaddition to controlling the pH of the buffer zone electrolyte to obtainthe desired product, temperature is also controlled Normally, it ismaintained at less than 105C, prefera bly being from 20 to 95C., morepreferably, 50 tc 95C. and most preferably, about 60 to C. or C Similartemperatures apply to the electrolyte in the an olyte and catholytecompartments. However, the pH 01 the buffer solution and catholyte aredifferent frorr those of the anolyte, being about 14, compared tr about1 to 5, preferably 2 to 4 for the anolyte. The tem perature of theelectrolyte may be controlled by recir culation of various portionsthereof, in the anolyte catholyte and buffer zones. Also, it is affectedby the proportion of feed to such zones and the temperature: thereof.Feeds will be regulated to obtain the desirer temperatures, previouslymentioned. Of course, wher the temperature cannot be loweredsufficiently by re circulation, refrigeration of the recirculatingliquid may also be utilized. For example, the feeds of water, brim andrecirculated electrolyte or mixtures of these enter ing the anodecompartment or any of the other com partments may be cooled about 5 to40C. below thei otherwise obtained temperatures or to about 10C. be foreadmissions to such compartment(s).

When the hypochlorite is being produced in the buffer compartment or amixture of hypochlorite and chlorate is being made therein thehypochlorite content may be converted to chlorate externally of the cellby addition of chlorine or other acidifying agent to lower the pH from 8to l l to the range of 6 to 75. preferably 6 to 7. The chlorine employedis chlorine produced in the cell. lt is a preferred acidifying agent forthis reason and because byproduct chloride can be reused. Whether thechlorate is made externally or internally or whether the hypochlorite isremoved for use. excess chlorine sent to the buffer zone is alsorecoverable and reusable. Similarly. if chlorate is recovered from theliquid product the aqueous medium may be returned to the buffer zone.preferably after removal of chloride. too.

The processes of this invention realize greatly improved currentefficiencies clue to their prevention of the wasteful production ofoxygen in the anolyte compartment. Anolyte pH is kept low. to preventoxygen release. by neutralization of hydroxyl ions and in the presentprocess the chlorine in the buffer solution diminishes hydroxyl in theanolyte markedly. Thus. chlorine current efficiencies of from 90 to 97%are obtainable. together with caustic current efficiencies of from 75 to859% or higher. Also. the caustic made is free of chloride. normallycontaining as little as (1.1 to 10 g./l. thereof The hypochloriteconcentration will normally be from 5U g/l. to its solubility limit andthe chlorate concentration produciblc. either in the cell or externalthereto. is lit) to 450 ga l. The sodium hydroxide concentration fromthe catholyte can be increased by feed ing dilute sodium hydroxide.recirculating sodium hydroxide solution previously taken off. increasingthe electrolysis time or diminishing the rate of caustic takeoffAlternatively. more concentrated caustic solutions may be made byevaporation of comparatively dilute solutions produced. When moreconcentrated caustic is made in the catholyte the hpochlorite orchlorate made in the buffer zone will also be more concentrated.

The present cells may be incorporated in large and small plants. thoseproducing hypochlorite or chlorate while also making from to L000 tonsper day of chlorine or equivalent and in all cases efficienciesobtainable are such as to make the process economically desirable. It ishighly preferred however. that the instal' lation should be located nearto and be used in con junction with a pulp bleaching plant. so that thehypochlorite or chlorate can be employed as a bleach or in theproduction of bleaching agent. eg. chlorine dioxide.

The following examples illustrate but do not limit the invention. Unlessotherwise indicated. all parts are by weight and all temperatures are inC.

EXAMPLE l To produce hypochlorite electrolytically and externallyconvert it to chlorate the apparatus illustrated in FIG. I is employed.with the electrolytic cell having steel walls. The anode compartment islined with polyester resin and the buffer compartment is lined withpolypropylene. The anode is of an expanded diamondshaped titanium mesh(l mm. in thickness and expanded to 50% open area with strand thicknessand width being equal coated with a mixture of ruthenium oxide andtitanium oxide U.l mm. thick. in a ratio of l:3. The titanium mesh iscommunicated with a positive direct current electrical source through atitaniumclad copper conductor. The cathode is of mild steel woven wiremesh 2.2 mm. in diameter and 6 X 6 to the inch and is communicated to anegative electrical source or sink by a copper conductor. The wallsseparating the anode and cathode compartments. and together with wallsof the cell. defining the buffer compartment. are of a cation-activepermselective mem brane manufactured by E. l. DuPont de Nemours &Company and sold under the trade name Nation. Char acteristics of suchmembranes are described in 21 Du- Pont New Product Information Bulletinof lO/l /69 under the title XR Pcrfluorosulfiniit' Acid Membranes. Thewalls of the membrane are seven mils thick (about 0.2 mm.) and it isjoined to a backing or supporting network of polytetrafluoroethylene(Teflon) filaments having a diameter of about 0.l mm. and arranged in ascreen or cloth form so that the area percentage of openings therein isabout 25%. The cross-sectional shape of the filaments is substantiallycircular and the membranes mounted on them are originally flat and arefused onto the screen or cloth by high temperature. high compressionpressing. with some of the membrane actually flowing around thefilaments during the fusion process to lock onto the cloth.

The material of the premselective membrane is a hydrolyzed copolymer ofa perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyletherv The hydrolyzed copolymer is of tetrafluoroethylene and ilnd hasan equivalent weight in the 900 to L600 range. about 1.250.

ln the electrolytic cell illustrated in H6. 1. for clarity ofpresentation and in accord with conventional cell illustrations. spacesare shown between the buffer compartment membranes and the electrodesbut in the practice of this experiment the electrodes are in contactwith the buffer membranes. with the flatter sides of the membranesfacing the contacting electrodes. The buffer compartment between them isV4 inch (6.4 mm.) wide, for minimum voltage drop at satisfactoryproduction rates and the intcrelectrode distance is essentially thesame. although gaps of 7/16 inch are also successfully used.

The anode compartment is filled with a saturated salt solution or brineand the cathode and buffer compartments are filled with water. initiallycontaining a small quantity of salt or brine to improve conductivity.Then the current is turned on and chlorine is fed to the buffercompartment to convert any sodium hydroxide transmitted thereto tosodium hypochlorite and sodium chloride. Chlorine is removed from theanode compartment and, in addition to being taken off for use or sale aschlorine. some thereof is fed to the buffer compartment and anadditional proportion is utilized to help to convert sodium hypochloriteto sodium chlorate externally of the cell. Hydrogen gas is removed fromthe cathode compartment and. after it reaches a satisfactoryconcentration, sodium hydroxide is also taken off from that compartmentand is essentially free of chloride ions. containing about l g./l. ofsodium chloride.

During operation of the cell the pH in the buffer compartment ismaintained in the range of 8 to ll, at about 10. to promote formation ofhypochlorite. Control of the pH in the buffer compartment is maintainedby adjusting the feed of chlorine and to some extent. water. The pH inthe anode compartment is held at about 4 and acidification control ismaintained by addition of small proportions of hydrochloric acid. Of

13 course, the pH in the cathode compartment is 14.

The solution of sodium hypochlorite and sodium chloride is conveyed fromthe electrolytic cell to a retention vessel from which it is pumpedcontinuously in a cycle through a reactor wherein the hypochlorite istreated with chlorine to produce sodium chlorate and more sodiumchloride. The mixture is drawn off from the retention vessel and thesodium chloride is subsequently separated from the sodium chlorate sothat the chlorate may be utilized in pulp bleaching without streampollution by the accompanying chloride.

In a modification of the described process means are provided forremoving sodium chloride from the circulating stream from the retentionvessel and chlorate liquor, essentially free of chloride is partlyreturned to the retention vessel through the reactor, where a smallproportion of sodium hypochlorite present therein is reacted withchlorine to produce additional chlorate, and another portion of thechloride-free chlorate is removed from the system, to be crystallized tosolid chlorate or to be employed as a chlorate liquor. Whencrystallized, the mother liquor is returned to the buffer compartment ofthe electrolytic cell. The following table describes the operation ofthe process (unmodilied) of this example in a number of variations ofthe described process.

EXAMPLE 3 The procedure of Example 1 is followed with the exception thatthe apparatus of FIG. 3 is employed and sodium chloride is continuouslyremoved from recirculating chlorate, which circulates through a chlorideremoval apparatus and also back to the buffer compartment. By thismethod chlorate is continuously removed from the holding vessel andchloride content is maintained low enough so that it does notcrystallize out in the cell or other portions of the apparatus.

EXAMPLE 4 Using a commercial size three-compartment cell like that ofFIG. 1 chlorate is formed externally at the rate of 0.42 ton per day ofsodium chlorate, at 95% conver- TABLE 1 EXAMPLES l-l l-2 l-3 [-4 1-5Average Anolyte NaCl Conc. (g./1.) 270.0 270.0 270.0 270.0 270.0 Av.Anolyte NaClO, Conc. (g./l.) 1 g./l. 1 gJl. 1 g./l. 1 g./1. l g./l. Av.Buffer Compartment NaOH Conc. (g./1.) 36.20 21.80 48.32 58.12 19.32 Av.Catholyte NaOl-I Cone. (g./I.) 339.80 238.40 325.68 395.36 236.88 Av.Catholyte NaClO, Conc. (g./l.) 1.3 2.95 0.80 0.45 265 Av. Catholyte NaClCone. (g./l.) 0.78 1.33 1.27 1.40 276 Av. Anolyte Flow Rate (I./min.)0.81 0.76 0.94 0.80 095 Av. Buffer Inlet Flow Rate (ml/min.) 20.0 19.519.8 Av. Buffer Exit Flow Rate (ml/min.) 19.1 19.5 19.0 20.0 15.2 Av.Catholyte Exit Flow Rate (l./min.) 0.151 0.115 0.082 0.054 0.438 AnolyteVolume in System (1.) 121.910 118.339 123.695 121.650 24.000 AnodeCompartment Volume (1.) 3.840 3.840 3.840 3.840 3.840 CathodeCompartment Volume (1.) 3.884 3.884 3.884 3.884 3.884 Buffer CompartmentVolume (1.) 0.520 0.520 0.520 0.520 0.520 Av. Anolyte Temperature (C.)72 72 64 81 95 Av. Anolyte pH 4.47 3.90 4.40 5.35 3.95 Av. CatholyteTemperature (C.) 68 63 54 79 81 Av. Current Density (a.s.i.) 1.204 1.0520.740 1.500 1.667 Av. Cell Volta c 5.163 4.943 4.798 5.407 4.894 AnodeNaCl E iciency 97.70 92.26 91.81 92.34 87.2. Anode Current Efficiencyfrom as analysis) 94.10 92.82 90.69 NaOH Accounted for (or NaOH Elciency, 'k) 98.36 92.46 99.56 97.27 91.38 NaClO, Efficiency (11) 95.4090.72 98.54 93.67 90.99 Operational Cell Time (hours) 23.00 19.00 22.5016.08 3.00

EXAMPLE 2 In the procedure described the feed of sodium chloride to theanolyte compartment is at about 25% sodium chloride concentration and inthe effluent from the anolyte the chloride concentration is about 22%.The chloride-free caustic is taken off from the cathode compartment andthe buffer compartment material is either employed as hypochlorite or,as illustrated in FIG. 1, is fed to a reactor and then to a holding tankequipped with means to lower the chloride concentration duringrecirculation. In the holding tank, wherein the pH is held at 6.5, thehypochlorite is converted to chlorate with a typical concentration andthat of this example being 430 g./l. of sodium chlorate, with 140 g./l.sodium chloride. In some runs as much as 500 g./l. of the chlorate andas little as 100 g./1. of the chloride are produced. The hypochloriteand chlorate produced sion, maintaining the buffer compartment pH atabout 10.5 and the reactor and holding vessel pH at about 6.5. Thecurrent is 90 kiloamperes and the current density is 2 amperes/sq. in.,at a direct current potential of 4.5 volts and at C., and the process iscontinuous. The chlorine feed to the buffer compartment is at the rateof 0.89 ton perday of the 3 tons per day of chlorine produced at theanode at 95% current efficiency. Sodium hydroxide produced is at a 25%concentration and is made at the rate of 2.28 tons per day. Sodiumchloride solution charged to the anode compartment is a 25% solution andthe concentration of sodium chloride in the effluent from thatcompartment is 22%.

EXAMPLE 5 Using acommercial apparatus like that of FIG. 2 andmaintaining the buffer compartmeill pH at 6.5, 0.4 ton per day of 0diumchlorate is made ill situ in the Blltfer compartmfilli at a conversionrate. A small Broportion (about of hypochlorite is present in theproduct. The pH is maintained by addition of more chlorine to thecompartment. Other conditions are the same as described in Example 4. Ina modification a batch process is employed with essentially the sameresults. When in place of the described membrane there are substitutedl8STl2S and 16ST13S RAI membranes of about twice the thickness of the XRperfluorosulfonic acid membranes employed in the other examples the samereactions are effected and the desired products also result. However, insuch cases it is noted that the RA] membranes are not as resistant tothe electrolyte and the products of electrolysis and do not last as longin use until replacement becomes desirable. This is especially true whenthinner membranes, such as those of 7 mil thickness are employed.

The invention has been described with respect to working examples andillustrative embodiments but is not to be limited to these because it isevident that one of ordinary skill in the art will be able to utilizesubstitutes and equivalents without departing from the spirit of theinvention or the scope of the claims,

What is claimed is:

l. A method for electrolytically manufacturing a hypochlorite whichcomprises electrolyzing an aqueous solution containing chloride ions inan electrolytic cell having at least three compartments therein, beinganode and cathode compartments and at least one buffer compartment, ananode, a cathode, at least one cation-active permselective membraneselected from the group consisting of a hydrolyzed copolymer of aperfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether,and a sulfostyrenated perfluorinated ethylene propylene polymer,defining a cathodeside wall of a buffer compartment between the anodeand cathode, an anode-side wall of a buffer compartment being defined bysuch a cation-active permselective membrane or a porous diaphragm, andsuch walls, with walls thereabout, defining anode, cathode and buffercompartments, while feeding gaseous chlorine into a buffer compartmentand regulating the rate of feed thereof and reaction conditions toproduce hypochlorite in the buffer compartment.

2. A method according to claim 1 wherein the permselective membrane(s)is/are of a hydrolyzed copolymer of tetrafluoroethylene and a sulfonatedperfluorovinyl ether of the formula FSO,CF,C- F OCF(CF )CF OCFXCF,,which copolymer has an equivalent weight of about 900 to 1,600.

3. A method according to claim 2 wherein the pH of the aqueous buffercompartment solution is maintained in the range of about 6 to 11, thetemperature thereof is less than about 105C. and the cell contains asingle buffer compartment.

4. A method according to claim 3 wherein the anode side and cathode sidewalls of the buffer zone are of the permselective membrane, which is ofa hydrolyzed c0- polymer of tetrafluoroethylene and a sulfonatedperfluorovinyl ether of the formula of claim 4, the membrane walls arefrom about 0.02 to 0.5 millimeter thick and the buffer solution pH isfrom 8 to 11.

5. A method according to claim 4 wherein the membranes are mounted on anetework of a material selected from the group consisting ofpolytetrafluoroethylene, asbestos, perfluorinated ethylene propylenepolymer, polypropylene, titanium, tantalum, niobium and noble metals,which has an are percentage of openings therein from about 8 to 80%.

6. A method according to claim 5 wherein the temperature is from to95C., the network is a screen or cloth of polytetrafluoroethylenefilaments having a thickness of 0.01 to 0.5 mm., being less than orequal to the thickness of the membrane mounted thereon and the areapercentage of openings in the screen or cloth is from about 10 to 70%.

7. A method according to claim 6 wherein the membrane walls are from 0.1to 0.3 mm. in thickness and the temperature of the electrolyte isregulated at least in part by the recirculation of compartment contents.

8. A method according to claim 1 wherein the hypochlorite made is sodiumhypochlorite and chloride ions are from sodium chloride.

I! i Ii UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.1 3,925,174 DATED December 9, 1975 INVENTOR(S) Jeffrey D. Eng 91: a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown beiow:

Column 1, line 67, "cationactive perselective" should read--cation-active permselective--;

Column 6, line 53, "2 to should read--2 to 3--;

Column 7, line 38, "into spaces" should read--into the spaces--;

Column 12, v

line 12, "Premselective" should read -perm selective--;

Column 14, line 56, "perday" should readper day---; Claim 2, line 7 "CFOCFXCF should read-CF OCF=CF2-;

Signed and Scaled this Twenty-third Day of November 1976 [SEALI' Arrest:

RUTI'I C. MASON C. MARSHALL DAN" 17 Commissioner ofParerm and Trademarks

1. A METHOD FOR ELECTROLYTICALLY MANUFACTURING A HYPOCHLORITE WHICHCOMPRISES ELECTROLYZING AN AQUEOUS SOLUTION CONTAINING CHLORIDE IONS INAN ELECTROLYTIC CELL HAVING AT LEAST THREE COMPARTMENTS THEREIN, BEINGANODE AND CATHODE COMPARTMENTS AND AT LEAST ONE BUFFER COMPARTMENT, ANANODE, A CATHODE, AT LEAST ONE CATION-ACTIVE PERMSELECTIVE MEMBRANESELECTED FROM THE GROUP CONSISTING OF A HYDROLYZED COPOLYMER OF APERFLUORINATED HYDROCARBON AND A FLUOROSULFONATED PERFLUOROVINYL ETHER,AND A SULFOSTYRENATED PERFLUORINATED ETHYLENE PROPYLENE POLYMER,DEFINING A CATHODE-SIDE WALL OF A BUFFER COMPARTMENT BETWEEN THE ANODEAND CATHODE, AN ANODE-SIDE WALL OF A BUFFER COMPARTMENT BEING DEFINED BYSUCH A CATION-ACTIVE PERMSELECTIVE MEMBRANE OF A POROUS DIAPHRAGM, ANDSUCH WALLS, WITH WALLS THEREABOUT, DEFINING ANODE, CATHODE AND BUFFERCOMPARTMENTS, WHILE FEEDING GASEOUS CHLORINE INTO A BUFFER COMPARTMENTAND REGULATING THE RATE OF FEED THEREOF AND REACTION CONDITIONS TOPRODUCE HYPOCHLORITE IN THE BUFFER COMPARTMENT.
 2. A method according toclaim 1 wherein the permselective membrane(s) is/are of a hydrolyzedcopolymer of tetrafluoroethylene and a sulfonated perfluorovinyl etherof the formula FSO2CF2CF2OCF(CF3)CF2OCF X CF2, which copolymer has anequivalent weight of about 900 to 1,600.
 3. A method according to claim2 wherein the pH of the aqueous buffer compartment solution ismaintained in the range of about 6 to 11, the temperature thereof isless than about 105*C. and the cell contains a single buffercompartment.
 4. A method according to claim 3 wherein the anode side andcathode side walls of the buffer zone are of the permselective membrane,which is of a hydrolyzed copolymer of tetrafluoroethylene and asulfonated perfluorovinyl ether of the formula of claim 4, the membranewalls are from about 0.02 to 0.5 millimeter thick and the buffersolution pH is from 8 to
 11. 5. A method according to claim 4 whereinthe membranes are mounted on a netework of a material selected from thegroup consisting of polytetrafluoroethylene, asbestos, perfluorinatedethylene propylene polymer, polypropylene, titanium, tantalum, niobiumand noble metals, which has an are percentage of openings therein fromabout 8 to 80%.
 6. A method according to claim 5 wherein the temperatureis from 60* to 95*C., the network is a screen or cloth ofpolytetrafluoroethylene filaments having a thickness of 0.01 to 0.5 mm.,being less than or equal to the thickness of the membrane mountedthereon and the area percentage of openings in the screen or cloth isfrom about 10 to 70%.
 7. A method according to claim 6 wherein themembrane walls are from 0.1 to 0.3 mm. in thickness and the temperatureof the electrolyte is regulated at least in part by the recirculation ofcompartment contents.
 8. A mEthod according to claim 1 wherein thehypochlorite made is sodium hypochlorite and chloride ions are fromsodium chloride.